The present invention relates to the field of pulleys. In particular, the invention relates to plastic pulleys with an insert molded bearing assembly comprising an outer race, an inner race, and a plurality of roller bearings seated between the outer and inner races. More particularly, the pulleys of the invention are adapted to have resistance to rotation of the axial bearing relative to the pulley. The present invention also relates to a method for manufacturing such pulleys. In the present invention, the terms “bearing,” “metal bearing,” “metal rolling bearing,” and “integral bearing” are interchangeable; and the terms “insert,” “rigid insert,” “bearing insert,” and “metal tubular insert” are interchangeable.
Plastic pulleys are known in the art as taught by U.S. Pat. Nos. 6,220,635; 6,200,513; 6,181,239; 6,120,401; 6,102,822; 6,090,001; 6,086,809; 6,032,635; 5,830,046; 5,782,709; 5,725,448; 5,724,930 and 5,176,580. Such plastic pulleys have heretofore been made by either molding-in a bearing, incorporated as an insert in the molding process, or by force fitting a bearing into a molded bore in the plastic part. For example, U.S. Pat. Nos. 4,468,210 and 4,473,363 describe examples of a plastic outer pulley body molded on a metal disc; and U.S. Pat. Nos. 4,913,688 and 5,476,423 describe examples of a two-step assembly wherein first a plastic outer pulley body is molded and then the metal insert is placed into the already molded pulley body.
Traditionally, when a pulley produced by the one-step process of injection molding the plastic pulley body about the metal bearing, the bearing can be physically altered to include one or more projections such as radially extending scallops or knurls on the outer periphery of the bearing.
Pulleys are also produced by the two-step process wherein a metal bearing is attached to a molded plastic outer pulley by forcing the bearing into the bore of the pulley. An adhesive is generally applied between the outer metal surface of the insert and the inner surface of the plastic pulley body to assist in bonding the bearing to the pulley body. In each of the methods, the bearing retention is less than desirable since the clamping force, which secures the bearing, is dependent upon the modulus of the plastic material and upon the surface of the outer bearing race. Typically, the outer bearing is smooth and made from material having relatively little affinity for the plastic pulley. Furthermore, asymmetric forces, which may occur during the molding process or during use, can distort the bearing to an out-of-round condition, thereby, shortening its useful life. For example, when a pulley is subjected to a heavy radial load from a belt, such as a v-ribbed belt, entrained therearound, the pulley and the bearing are slightly deformed and, since the deformation patterns of the plastic pulley and the insert are different, a slight clearance is defined between the inner surface of the pulley and the outer surface of the bearing. This clearance becomes wider as the radial force from the belt increases and over time, results in the wear of the fit surfaces of the pulley and the outer surface of the bearing, thus decreasing the joining force between the pulley and the bearing.
U.S. Pat. No. 4,822,111 to Albert teaches a two-part wheel assembly, such as used for wheelbarrows, wherein the two parts are joined together and a plain type anti-frictional bearing, usually molded plastic such as nylon, modified with a material, which provides good thermal, mechanical and bearing properties.
All journal bearings, e.g., engine main bearings and connecting rod bearings, incorporate some type of anti-rotation device, usually in the form of an outwardly-deflected tab which mates with a relief in the bearing cavity. It is also generally known to knurl or otherwise physically texture the outer surface of the outer race of a rolling-element bearing to be molded or captured in a bearing cavity. Also, it is generally known to glue the outer race of the bearing into its cavity, either directly in the part or in a separate shell insert. The use of a separate shell insert is disclosed in copending commonly assigned U.S. patent application Ser. No. 10/654,567, filed Aug. 29, 2003, the contents of which are incorporated herein by reference thereto. However, the process of bonding the bearing assembly and/or an insert to the pulley with an adhesive or glue presents undesirable clean-up and environmental problems generated by the large amounts of adhesive or glue employed. Accordingly, there is a need, particularly in the area of an endless drive belt of a vehicle accessories drive system, for a plastic pulley with a metal bearing that may be easily manufactured without the problems associated with prior art pulleys.